This invention relates to an apparatus for measuring the droplet size distribution of very small particles.
This type of an apparatus is now being developed which measures the droplet size distribution of, for example, droplets of vapor in a turbine or droplets of fuel jetted into a combustion chamber, through the utilization of Mie scattering. Here the droplet size distribution means a relative distribution in the number of particles for a respective droplet size or diameter, i.e., a probability density function.
According to "Mie scattering" theory, the scattered light intensity i(D,.theta.) in a certain angular direction .theta. (at a scattering angle) when very monochromatic light beams, such as laser light, strike a spherical particle with a diameter D can be calculated. The scattered light intensity I(.theta.) of a group of spherical particles of various sizes can be expressed as a sum of respective scattered beams i(D,.theta.) on one particle of the diameter D, as set out below. EQU I(.theta.)=.intg.i(D,.theta.)n(D)dD (1)
where n(D) denotes the droplet size distribution.
A method for measuring the droplet size distribution with the use of the principle of Equation (1) is called as the forward scattering method. This method is one type of the indirect measuring method which comprises:
(1) a first step of irradiating a group of particles with monochromatic parallel laser beams and measuring the angular distribution of the irradiation beams scattered on the group of particles--hereinafter referred to simply as the intensity distribution of the scattered beams; and
(2) a second step of evaluating the droplet size distribution n(D) from the intensity distribution of the measured scattered beams I(.theta.) with the use of Equation (1).
FIG. 5 shows a conventional apparatus for measuring the droplet size by the aforementioned method. In this apparatus, output light 60 from a light source (not known), such as a laser is conducted via illumination optical fiber 3 into a scattering zone A and directed at the group of particles 6 in the scattering zone A. Beams 61 scattered at the respective scattering angles are conducted to photodetectors (not shown) via optical fibers 7a, 7b, 7c and 7d to measure the intensity distribution I(.theta.) of the scattered light. These optical fibers 7a, 7b, 7c, and 7d are arranged equidistant from the center P of the scattering zone A and for predetermined scattering angles.
In this apparatus, however, on account of the optical fibers 7a, 7b 7c, and 7d arranged at the predetermined scattering angles the optical fiber 7c, for example, receives not only the beams scattered at the scattering angle .theta..sub.f but also beams scattered on the particles occupied in a broader range as indicated by broken lines 63 in FIG. 5. As a result, the photodiode connected to the optical fiber 7c will receive the scattered beams at other than the scattering angle .theta..sub.f as well. Stated in a stricter way, .theta. in Equation (1) fails to indicate the set angles of optical fibers 7a, 7b, 7c, and 7d. However, no problem arises in the cases where the scatterinq path lenqth L in the scatterinq zone A is shorter. The scattering path lenqth L may be made shorter in the case of measuring a larger droplet size since the intensity of the scattering light is adequate. For the smaller size of particles to be measured, on the other hand, the intensity of the scattering light is very small and, in order to increase the number of particles in the scattering zone A, it is necessary to increase the scattering path length L, failing to express the intensity distribution of the scattering light with Equation (1).